Although the invention is herein described with reference to systems for copolymerization of propylene and ethylene, it will be understood that the invention can be readily applied to the copolymerization of other alpha-olefin monomer combinations such as propylene-butene, propylene-hexene and also terpolymer systems produced from three or more olefinic monomers.
"Propylene impact copolymers" are polymers which are composed of a polypropylene homopolymer phase which is intimately mixed with one or more ethylene-propylene copolymer phases. This mixture results in a product which has good impact resistance and good stiffness.
Impact copolymers are typically produced by two or more reactors in series. The first reactor typically produces polypropylene homopolymer which is then fed to a second reactor. Alternatively, the first reactor can be used to produce random copolymer which would then be fed to the second reactor. In the second reactor (and subsequent reactors, if any) the reactant composition is varied such that copolymers with varying fractions of ethylene and propylene are produced in each reactor and intimately mixed with the polymer from the previous reactors.
Typically, the reaction in the reactors, which can be gas phase reactors, is catalyzed by a transition metal catalyst. In most cases the transition metal is titanium.
In general, the equipment for producing propylene impact copolymers is conventional equipment such as two or more reactors, heat exchangers, compressors, discharge systems and piping connected to the various equipment.
Unfortunately, however, during normal operations, the surfaces of the tubes of the heat exchanger or cooler tend to foul with undesirable polymer deposits. These deposits tend to reduce the heat exchanger capability in cooling the recycled gas which removes the heat of reaction, and also it increases the pressure drop across the heat exchanger, which adds to the load on the cycle gas compressor. Because of increasing pressure drop and/or decreased heat exchanger capability the reactor must be shut down within a short time for cleaning.
According to EPA publication No. 0282929 there is disclosed a method for producing a Propylene-alpha-olefin block copolymer without polymer agglomeration in the reactor, by supplying at least one compound selected from the group consisting of an aromatic carboxylic acid ester, a phosphorous ester, an unsaturated dicarboxylic acid diester, amine compound and an amide compound, to the reactor or recycle line. The preferred carboxylic acid ester can be ethyl benzoate and ethyl ethoxybenzoate. Unfortunately however polymer build up in heat exchangers is still a problem when using the method disclosed in the EPA publication 0282929.